FIG. 28 shows a schematic arrangement in a conventional transparent document image reading apparatus. Referring to FIG. 28, a transparent document 142 such as a positive film, negative film, or the like placed on a platen glass 141 is illuminated with light emitted by a transparent document illumination lamp 144 via a diffusion plate 143 set above the document, and light transmitted through the transparent document 142 is guided to a CCD 150 via a mirror 147, inverted-V mirrors 148, and imaging lens 149. The light is converted by the CCD 150 on which a large number of solid-state image sensing elements line up into an electrical signal, thus obtaining an image signal in the main scan direction.
In this case, image reading in the sub-scan direction is done by mechanically moving the transparent document illumination lamp 144 and mirror 147 in the sub-scan direction with respect to the transparent document 142 while maintaining an identical velocity and phase, and making the inverted-V mirrors 148 track at the half scan velocity in the sub-scan direction so as to maintain a constant optical path length (conjugate relationship) from the transparent document 142 to the CCD 150. In this way, a two-dimensional image is read in combination with the process in the main scan direction.
The aforementioned transparent document image reading apparatus can read a so-called reflecting document which is described on an opaque material and is illuminated with light so as to process the light reflected by the material. In this case, a reflecting document is placed in place of the transparent document 142, and is illuminated with a direct light beam emitted by a reflecting document illumination lamp 145, which is turned on in place of the transparent document illumination lamp 144, and with a light beam reflected by a reflector 146. The light reflected by the reflecting document is read by the CCD 150, thus forming an image in the main scan direction as in the transparent document.
Especially, as a color reading method, a 3-line color image reading method is prevalent. That is, the reflecting document illumination lamp 145 uses a lamp having white spectral characteristics, and the CCD 150 uses a 3-line type CCD having R, G, and B color filters. Three colors (R, G, and B) of image information are simultaneously read by a single scan, and R, G, and B color signals on an identical line are superposed by an image processing circuit, thus forming a color image.
In order to correct any defects on an image due to dust, scratches, and the like on a transparent document in the aforementioned transparent document image reading apparatus, the only effective method is to retouch them using image edit software after the image is read. For this reason, a very long time is required to correct such defects.
In recent years, as such transparent document image reading apparatus, an image reading apparatus having a so-called dust/scratch removal function of detecting dust present on a transparent document and scratches on a film surface (such detection will be referred to as “dust/scratch detection” hereinafter), and removing the influences of such dust and scratches by an image process has been developed.
FIG. 29 shows a conventional image reading apparatus 1 having a dust/scratch detection function. The same reference numerals in FIG. 29 denote the same parts as in FIG. 28, and a detailed description thereof will be omitted.
Referring to FIG. 29, reference numeral 151 denotes an infrared lamp which comprises an LED having an emission intensity peak at a wavelength of about 880 nm.
FIG. 30 is a block diagram showing the functional arrangement of a dust/scratch remover 2 for implementing dust/scratch removal using image data obtained by the image reading apparatus 1. Referring to FIG. 30, reference numeral 21 denotes an interface (I/F) for inputting image data read by the image reading apparatus 1; 22, an image memory for storing an image read using the transparent document illumination lamp 144 or reflecting document illumination lamp 145 (to be referred to as a “normal image” hereinafter); 23, an infrared image memory for storing an image read using the infrared lamp 151 (to be referred to as an “infrared image” hereinafter); 24, a threshold value holding unit for holding a predetermined threshold value; 25, a dust/scratch detection unit; and 26, a dust/scratch correction unit.
FIG. 31 shows the spectral intensity distributions of the transparent document illumination lamp 144 and infrared lamp 151, and the characteristics of these lamps are represented by the solid and dot-dash-curves, respectively. FIG. 32 shows the spectral transmittance characteristics of cyan, yellow, and magenta dyes of a general negative/positive film, and the peak wavelength (about 880 nm) of the spectral intensity distribution of the infrared lamp 151. As is apparent from FIG. 32, most light components emitted by the infrared lamp are transmitted through a general color film irrespective of an image on the film since all dyes have very high transmittance at about 880 nm.
The transparent document reading operation including dust/scratch removal will be explained in detail below with reference to the flow chart shown in FIG. 33.
In step S10, the reflecting document illumination lamp 145 and infrared lamp 151 in FIG. 29 are turned off, and the transparent document illumination lamp 144 is turned on. At this time, an illumination light beam emitted by the transparent document illumination lamp 144 is uniformly diffused by the diffusion plate 143, and that diffused light beam is transmitted through the transparent document 142. The transmitted light beam passes through the mirror 147, inverted-V mirrors 148, and imaging lens 149, and is projected onto the CCD 150. An image projected onto the CCD 150 is converted into an electrical signal, which is temporarily stored in the image memory 22 via the I/F 21 in FIG. 30.
In step S20, the reflecting document illumination lamp 145 and transparent document illumination lamp 144 in FIG. 29 are turned off, and the infrared lamp 151 is turned on. An illumination light beam emitted by the infrared lamp 151 with the characteristics shown in FIG. 31 is uniformly diffused by the diffusion plate 143. The diffused light beam is transmitted through the transparent document 142, and passes through the mirror 147, inverted-V mirrors 148, and imaging lens 149. The light is then projected onto the CCD 150. Hence, the illumination light beam emitted by the infrared lamp 151 is transmitted through the transparent document 142 irrespective of an image (exposure) of the transparent document 142 such as a negative film, positive film, or the like, as shown in FIG. 32, and an image of dust, scratch, or like, which physically intercepts the optical path, is projected onto the CCD 150 as a shadow. The infrared image projected onto the CCD 150 is converted into the electrical signal, which is temporarily stored in the infrared image memory 23 via the I/F 21 in FIG. 30.
In step S30 and subsequent steps, dust/scratch detection and correction are executed. The principle of dust/scratch detection will be described in detail below.
FIGS. 34A to 34C illustrate the relationship between dust or the like, and the gray levels of images read using the transparent document illumination lamp 144 and infrared lamp 151, which are plotted in the main scan direction. In FIG. 34A, reference numeral 181 denotes a positive film; and 182, dust on the positive film 181. FIG. 34B shows the gray level obtained when a corresponding portion in FIG. 34A is read using the transparent document illumination lamp 144. The gray level assumes a lower value as an image becomes darker. The gray level of the dust portion 182 is low irrespective of an image on the positive film. FIG. 34C shows the gray level obtained when the portion in FIG. 34A is read using the infrared lamp 151. The dust portion 182 has low gray level since no infrared light is transmitted through there, and a portion other than the dust 182 has a nearly constant level 183 since infrared light is transmitted through there. Hence, a threshold value 184 is set at a gray level lower than the level 183, and a defect region 185 formed by dust can be detected by extracting a portion having a gray level equal to or lower than the threshold value 184.
The threshold value 184 is held in advance in the threshold value holding unit 24. Therefore, the dust/scratch detection unit 25 reads out this threshold value 184 from the threshold value holding unit 24, and compares it with infrared image data in turn in step S30, thus detecting the defect region 185.
If the infrared image data is smaller than the threshold value 184 (NO in step S30), the influence of dust 182 is eliminated by executing, e.g., an interpolation process of the defect region 185 based on a normal region around it in step S40. The comparison process is executed for all infrared image data, and when any defect region is detected, the corresponding normal image data undergoes interpolation (step S50).
However, the aforementioned prior art cannot normally detect a defect portion or erroneously detect even a normal portion as a defect portion due to insufficient detection precision. That is, the nearly constant level 183 of infrared rays that have been transmitted through the transparent document largely varies due to light amount errors of the infrared lamp 151, transmission errors depending on the type of color film at the emission wavelength of 880 nm of the infrared lamp 151, and sensitivity errors of the CCD 150 at the emission wavelength of 880 nm. For this reason, if the threshold value 184 is set as a fixed value, the level 183 assumes a value higher than the threshold value 184, and even a normal image portion is detected as a defect portion, or the threshold value 184 defines a gray level much lower than the level 183, and a defect region cannot be accurately detected.